Stabilizer arrangement for cyclotrons and the like



Oct. 28, T952 R. R. WEEKS 2,616,042

STABILIZER ARRANGEMENT FOR CYCLOTRONS AND THE LIKE Filed May 17, 1950 2 SHEETS-SHEET 1 FIG. L

1 Snnentor Xo Ez lFAY WEE/r5 Gttorneg Patented Oct. 28, 1952 UNITED STATES PATENT OFFICE STABILIZER ARRANGEMENT FOR CYCLOTRONS AND THE LIKE Robert Ray Weeks, Upton, N. Y., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application May 17, 1950, Serial No. 162,493

16 Claims. 1

This invention relates to cyclotron systems, and more especially to stabilization and control thereof.

A principal object of the invention relates to an improved arrangement for stabilizing the excita tion and operation of the dee electrodes of a cyclotron, or similar device, employing a plurality of large size electrodes which are designed to be electrically balanced with respect to a ground or other fixed reference potential.

Another object is 'to provide a novel arrangement for stabilizing the excitation frequency of the dee electrodes of a cyclotron or similar device.

Another object is to provide a novel arrangement for maintaining the dee electrode of a cyclotron or similar device electrically balanced with respect to a predetermined peak voltage reference level or ground plane.

A feature of the invention relates to a novel combination of circuits and apparatus for simultaneously stabilizing the frequency and ground reference balance of the dees of a cyclotron.

A further feature relates to an improved combination of manual and automatic controls, whereby the operating resonance frequency of a cyclotron can be accurately controlled from a remote point, while maintaining the required fre quency stability and ground reference balance of the does.

A further feature relates to a control system for cyclotrons whereby greater speed of response is obtainable While searching for the optimum adjusted relation between the magnetic field strength, and the dee excitation frequency. Furthermore, where a desired magnetic field pattern has been ascertained, it is possible to maintain the field-producing current and magnetic flux distribution fixed, while varying the dee excitation frequency to maximize the cyclotron beam. This provides an improved tuning procedure which renders possible closer approach to the theoretical energies arrived at by mathematical computation. v

' A still further feature relates to the combination of a cyclotron with a specially-designed servo-motor system, whereby two follow-up voltages are generated, each corresponding respectively to the frequency deviation of the dee eX- citation and to the electrical unbalance of the dees. In accordancewith this feature, one voltage is applied to cause the pair of motors to be turned respectively in the same sense, while the other voltage is applied to cause the motors to turn simultaneously in opposite senses.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved cyclotron system.

Other features and advantages not particularly enumerated, will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing which shows, by way of example, a cyclotron system embodyingthe inventive concepts,

Fig. l is a schematic plan view of a cyclotron according to the invention.

Fig. 2 is a schematic side view of the cyclotron of Fig. 1.

Fig. 3 is a schematic wiring and block diagram of a complete control system according to the invention.

Referring to Figs. 1 and 2, there is shown in schematic form, a typical cyclotron comprising the usual separated hollow dees or electrodes Iii, H, which are suitably mounted within an evacuated enclosing chamber l2. A suitable ion developing gun [3 projects the ions or other heavy charged particles into the space between the dees, which are located between the north and south pole pieces 4, l5, of a suitably energized electromagnet. The dees are provided with respective lead-in members insulatingly sealed through the wall of chamber l 2 and connected via a transmission line It to any well-known radio frequency osc llator-generator H, which, for example, may generate a dee excitation frequency selectable in the range of for example 10 to 14 megacycles per second. Since the details of construction of the remaining elements of the cyclotron are wellknown, further detailed description is not deemed necessary herein but reference for that purpose may be madeto Electronics, first edition, by Millman 82 Seeley. pages 87 to 90, published by McGraw-Hill Book Company, Inc., New York, 1941.

Because of the relatively large size of the dees, they have an inherent capacity to ground schematically represented by the capacitances l8, l9, and as a result of which there is a virtual direct current ground between the dees. The tank circuit of the oscillator ii is schematically represented by the dotted rectangle 29, and includes the inductance 2|, the capacitances it, I :i, and two additional adjustable condensers 22, 23. Each of these condensers has its adjustable element connected to a respective two-phase servomotor 24, 25, through respective gear trains 2B, 21, for purposes to be described hereinbelow.

Located in coupled relation to the tank circuit is a small pick-up loop or probe 28. It has been found that in order to achieve the optimum results from a cyclotron, it is necessary to maintain the excitation of the dees at a constant selected frequency, and also to maintain the dees equally and symmetrically balanced, or at a predetermined unbalance, in an electrical sense, with respect to ground or other fixed reference potential. Undesired variations of these factors may take place in the conventional cyclotron arrangements as a result, for example, of thermal changes which affect the physical dimensions of the dees and the related structural parts, or as a result of the introduction of probes for various monitoring or measuring purposes. Furthermore, it is highly desirable to be able to adjust the frequency of the dee excitation from a convenient remote point, such for example as from a central console control panel or the like.

Accordingly, there is provided a comparison oscillator 23, which can be manually controlled, for example by the calibrated knob 30 at the control panel, to generate any selected frequency in the range 26.4 to 30.4 megacycles. The signal from oscillator 29 is fed to a frequency mixer device or network 3! of any well-known kind. to which is also fed the signal picked up by member 28. If for example, the dees are operating at a frequency of megacycles, then oscillator 29 can be set so as to generate 26.4 megacycles. Mixer 3| selects the difference frequency, for example 16.4iAf megacycles. This diiference frequency signal is then applied to any wellknown limiting amplifier 32 which limits the amplitude of the signal from mixer 3|, so as to maintain the loop gain at a constant value. If the cyclotron oscillator system should change in frequency undesirably, for example to 11 megacycles per second, the output of the mixer would then be 15.4 megacycles per second instead of 16.4 megacycles per second.

There is also provided a reference generator 33 which is capable of generating, for example, a two-phase 60-cycle current, one phase being represented by the value Er sin (wt+QO), the other phase being represented by the value Er sin wt. The output of generator 33 is at a constant frequency and is applied to a modulator 9, together with the signal from the amplifier 32. Thus the output wave of amplifier 32 is modulated by this constant frequency, and the modulated wave is fed to a detector-discriminator 34. The output of this discriminator therefore is an alternating current Whose amplitude is proportional to the undesired change in frequency of the cyclotron oscillator as indicated by the value AC'KE'KA sin (wt-l-90), wherein the elements KE and KA respectively are constants of the modulator, and AC is a factor proportionate the drift in frequency of the cyclotron oscillator. Thus if the cyclotron oscillator is oscillating at the desired 10 megacycles per second, the element AC=0, and the discriminator 34 gives no alternating current output. However, if the cyclotron oscillator has drifted by one megacycle, the discriminator will produce an alternating current output proportional to this error.

The output from discriminator 34 is applied in parallel to a two-channel amplifier 35 and to a similar two-channel amplifier 36, and the outputs of these amplifiers are applied respectively to one of the phase windings of the corresponding servo-motor 2d, 25. The other 90 phase Winding of each of these motors is excited by the voltage wave Er sin wt derived from generator 33. Since the two voltages applied to each motor are out of phase, each motor will turn in the same direction or sense, thus turning the corresponding adjustable element of the respective condenser 22, 23. These motors therefore keep turning until the output of amplifier 35 becomes zero. If the change in frequency of the cyclotron oscillator had been an increase, in order to bring the cyclotron system back to the desired frequency, the motors 2t, 25, turn the condensers 22, 23, to increase both of their capacitances, thus causing a lowering of the oscillator frequency. If the drift in frequency had been a decrease, then the motor 24 and 25 would be rotated in a direction which simultaneously decreases the capacitance of the condensers 22 and 23. In this manner the dees are maintained excited at the fixed desired frequency.

For the purpose of maintaining both dees equally and symmetrically electrically balanced with respect to ground, the opposite terminals 50, 4|, of the tank circuit are connected to a balance discriminator 31 which, for example, may consist of two voltmeters connected in opposed or differential arrangement so as to give at the output of the balance discriminator a direct current voltage which is proportionate to the amount of electric unbalance at each of the terminals 40, 4|, with respect to ground. This direct current signal representing the unbalance, is applied to a modulator 38 to which the signal Er sin (wt-l-QO") is also applied. This in effect adds a 60-cycle reference voltage from source 33 to the direct current output of discriminator 31 as represented by the expression AC-KB-Ks sin (wt+90) This latter signal is then amplified in the twochannel amplifier 3B and applied to motor 24. This unbalance signal is also fed to the input of the two-channel amplifier 35 but after having passed through a phase shifter network 30.

Consequently if electrical unbalance occurs at the terminals 40, 4|, one motor rotates to change the capacity 22 in one sense, while the other motor rotates to change the capacity 23 in the opposite sense. For example if the unbalance of the dees is such as to render terminal 40 higher in potential than terminal 4|, the motor 24 turns in the proper direction to increase the capacitance of condenser 22, while motor 25 simultaneously rotates in a direction to decrease the capacitance of condenser 23. The motors continue rotating until both terminals 40, 4|, are brought back to the balanced electrical condition. It will be observed therefore that the frequency stabilizing means and the direct current balancing means work simultaneously, and the motors 24 and 25 respond to the additive results of the two corrections.

With the foregoing described arrangement the various control functions are obtainable over a 9-to-.-1 range of dee voltage. Thus the resonant frequency of the dee system can be automatically maintained within :1 kilocycle of the reference oscillator 29, with a stability of better than 75 parts per million for a 24 hour period. Automatic control is maintained notwithstanding capacity changes resulting from change in positioning of the dees, thermal changes in the sizes of the dees, introduction of probes, or other disturbances, provided these disturbances are not so great as to exceed the range of tuning of the condensers 22, 23, corresponding to an amount, forexample, of 1-0.4 megacycle from the median portion. If desired, the condensers 22 and 23 may be shunted respectively by manually controllable condensers so as to locate the system initially at its mid-point, and to determine the range of control of the system, in which case the condensers 22 and 23 may be considered as automatically-controlled trimmer condensers.

Also with this arrangement, the voltages of the two dees with respect to ground canbe automatically maintained equal within approximately 2%. Single-ended disturbances will be compensated within the range of the condensers 22, 23, and the range can be extended by the manuallycontrolled shunt condensers as above mentioned. This feature assures a minimum power expendi ture in obtaining a given dee-to-dee voltage, and enables equality of grid drive for the oscillator tubes in the oscillator ll, thus resulting in equal loading of the two oscillator tube banks which are usually employed. in the cyclotron oscillator. Furthermore, precession of the cyclotron beam is minimized by maintaining a symmetrically fringing flux between the lips of the dees. This also helps to maintain homogeneity of the energy in the cyclotron beam.

It will be clear from the foregoing that by means of the knob 36, the frequency of the cyclotron oscillator can be varied within the range of the condensers 22, 23, and this knob can be operated by remote control at a suitable console panel. The cyclotron oscillator automatically follows the manual change in the comparison oscillator 29 within the range of the condensers 22,

23. By this arrangement, the actual operating frequency of the cyclotron oscillator can be di rectly displayed at the console panel. If the condensers 22, 23, at initial equilibrium are at midscale position, the previously-mentioned 10.4 megacycle range may be swept by adjustment of knob 30. This feature is very useful since it provides greater speed of response when searching for the cyclotron resonance condition between magnetic field and oscillator frequency, since less time lag is obtainable than when searching with the magnetic field alone due to the large amount of energy stored in the field.

From the foregoing it will be seen that the system as a whole, employs two main interlocked follow-up or servo loops which are somewhat unusual in that two error signals are utilized in either or both of the two output devices as required. Two minor loops are used in performing the functions of the main loops. The first minor loop is integral with the balance discriminator 3'! and is, in effect, the normalizing device. The second minor loop assures constant input to the mixer in the frequency error channel in View of the changing power level of the cyclotron oscillator. This minimizes spurious frequency error responses.

While one particular organization of apparatus has been disclosed herein, various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a system of the type described, the combination of a space discharge device having a pair of electrodes for operation at a selected frequency and with an electrically symmetric balance with respect to a predetermined fixed base potential, a source of oscillations for exciting said electrodes at said frequency, another source of comparison frequency, means to compare the frequency of both sources to derive a signal representing departure of the first source from its selected frequency, frequencyadj'usting means for the first-mentioned source, follow-up circuits for operating said frequency adjusting means under control of said signal to restore the first source to its selected frequency, and means to control said follow-up circuit to maintain said electrodes in said balanced condition, the last-mentioned means including a follow-up motor and circuits including a frequency discriminating detector for controlling said motor by said signal.

2. In a system of the type described, the cor. bination of a space discharge having a pair of electrodes for operation at a selected frequency and with an electrically symmetric balance with respect to a predetermined fixed base potential, 2. source of oscillations for exciting said electrodes at said frequencies, means including a comparison frequency source and a mixer to derive a first error signal corresponding to departure of said source from said selected frequency, means in cluding a discriminator for producing a balance signal proportionate to the relative unbalance of said electrodes with respect to said base potential to derive a second error signal corresponding to the departure of said electrodes from electrical balance, frequency adjusting means for said source, and follow-up circuits responsive to both said error signals for automatically adjust ing said frequency adjusting means and for automatically maintaining said electrodes electrically balanced, the last-mentioned means including a follow-up motor and a frequency discriminator detector to which both said error signals are applied, and circuit connections for controlling said motor by the output of said detector.

3. A system according to claim 2, in which said electrodes are of substantial area and respectively have substantial inherent capacitances to ground, and said last-mentioned means maintains said electrodes automatically balanced with respect to ground.

4. In a system of the type described, a space discharge device having a pair of electrodes, a gun for producing a beam by charged particles such as electrons, a magnetic field for controlling the trajectories of said charged particles: a source of excitation frequency for said electrod s, adjustable elements for controlling the frequency of said excitation and also the electrical balance of said electrodes with respect to a fixed base potential, '2. source of known comparison frequenc-y, means to compare both sources to derive an excitation frequency error signal, means including a balanced discriminator and a modulator to produce a signal representing relative unbalance of said electrodes with respect to said base potential to derive from said electrodes a balance error signal, and follow-up circuits to which both said signals are applied to maintain said electrodes excited at said selected frequency and. to maintain them symmetrically balanced with respect to said base potential.

5. A system according to claim 4, in which said source of excitation frequency comprises a high frequency oscillator having a tank circuit con stituted in part of said electrodes, and each of said adjustable elements comprises an adjustable capacitor each connected respectively between one of said electrodes andground.

6. In a system of the type described, a space discharge device having a pair of large area electrodes with substantial capacity to ground, an excitation oscillator, means connecting said device'as part of a frequency-determining circuit to said oscillator, adjustable capacitors connected to said electrodes to balance them electrically with respect to ground, a reference oscillator to known frequency, electric motor means connected respectively to said capacitors to adjust them, means to compare the frequency of the reference generator with the frequency of the excitation oscillator to derive a frequency drift error signal, means including a balance discriminator connected to said electrodes to derive a balance drift error signal proportionate to the relative unbalance of said electrodes with respect to ground, and electric circuits for controlling said motor means by both said error signals.

7. In combination, a cyclotron or similar device, an excitation oscillator for the dees of the cyclotron, said oscillator having: a frequencydetermining tank circuit constitutedin part by the inherent capacitance of said dees to ground, a pair of adjustable capacitors connected re spectively between said dees and ground, a balance discriminator coupled to said dees, a reference oscillator of known frequency, a pair of polyphase alternating current servo-motors each geared respectivey to the adjustable element of a correspondin one of said capacitors, a source of alternating current, means to energize the corresponding phases of thetwo motors directly from said alternating current source, means to derive a quadrature phase current from said alternating current source, means to derive from said reference oscillator and said excitation s.- cillator a frequency error signal, means'to mod ulate said error signal with said quadrature phase current, and means to apply. said modulated signal to the other corresponding phases of the two motors for the purpose set forth.

8. In combination, a cyclotron or similar device, an excitation oscillator for the dees of the cyclotron, said dees being at virtual. ground potential but direct current insulated from ground, said oscillator having a frequency-determining tank circuit constituted in part of said dees, a pair of adjustable condensers each connected between one of said dees and ground, said condensers being normally adjusted to balance said dees electrically with respect to ground, a balance discriminator coupled to said dees, a reference oscillator of known frequency, a pair of polyphase alternating current motors for adjusting said condensers respectively, means to generate two phase-quadrature voltages of the same frequency, means to derive a frequency deviation signal representing the difference between the frequency of said excitation oscillator and the frequency of said reference oscillator, means to intermodulate one phase of said alternating current with said frequency deviation signal to produce a desired modulation product, means to apply said modulation product and the other phase of said alternating current to both of said motors, means to derive an unbalance signal from said discriminator, means to intermodulate said unbalance signal with said one phase of said alternating current to derive another modulation product, means to apply said second modulation product to one motor, and means to apply said modulation product after 180 phase shift to the other motor.

- 9; In combination, a cyclotron or similar device, an excitation oscillator for the cyclotron dees, means including a master oscillator and a mixer to produce a first signal representing a frequency deviation of said oscillator excitation, means including a balance discriminator connected to said dees to produce a second signal representing electrical unbalance of said dees with respect to ground, condenser means coupled to said dees for controlling the excitation frequency of the oscillator and for balancing the dees to ground, servo-motor means geared to said condenser means, means to apply said first signal to operate said motor means and said condenser means to reduce said frequency deviation to substantially zero, and means to apply said second signal to operate said motor means and said condenser means to maintain said balance.

10. The combination according to claim 9 in which said condenser means comprises a pair of adjustable condensers each connected between a respective one of the dees and ground, said motor means comprising a pair of polyphase alternating current motors geared respectively to said condensers, means to apply said signals to control the operation of the motors and including follow-up circuits controlled simultaneously by said first and second signals.

11, Cyclotron apparatus comprising a cyclotron, a pair of adjustable condensers connected to the cyclotron dees to adjust the cyclotron excitation frequency and to balance the dees with respect to ground, a pair of motors each geared respectively to said condensers to adjust the same, a source of alternating current having two phases in quadrature, means to apply one of said phases directly to said motors, means including a balance discriminator connected to said dees to derive a balance deviation signal proportional to the unbalance of said dees with respect to ground, means including a master oscillator and a mixer to derive an excitation frequency deviation signal, means to modulate said frequency deviation signal and said other phase to produce a first modulation product, means to modulate said balance deviation signal with said other phase to derive a second modulation product, means to shift the phase of said second modulation product by means to apply the first modulation product and the 180 phase-shifted second modulation product to one of said motors, and means to apply the first modulation product and the second modulation product to the other motor to cause said motor simultaneously to adjust said condensers in the same sense to control the excitation frequency of the cyclotron and to control said condensers simultaneously in the opposite sense to balance the cyclotron dees with respect to ground.

12. Cyclotron apparatus comprising a pair of adjustable condensers connected between each cyclotron dee and ground, a pair of motors, means to derive a balance deviation signal, means to derive an excitation frequency deviation signal, and follow-up loop circuits interconnecting said motors with said condensers and controlled simultaneously by both of said signals to sta bilize the excitation frequency at a selected value and to balance the cyclotron dees electrically with respect to ground. v

13. Cyclotron apparatus according to claim 12 in which the means for deriving the said frequency derivation signal comprises a comparison oscillator having means to adjust its frequency and thereby to control the excitation frequency of the cyclotron dees.

14. In a system of the type described, a space discharge device having a pair of electrodes, a gun for producing a beam of charged particles such as electrons, magnetic field means for controlling the trajectories of said charged particles; a source of excitation frequency for said electrodes which electrodes have substantial inherent capacity to ground, adjustable capacitors connected respectively to said electrodes for symmetrically balancing them with respect to ground, a balance discriminator connected symmetrically to said electrodes and producing a balance signal proportionate to the unbalance of said electrodes to ground, a source of comparison frequency, mixer means to compare said comparison frequency with said excitation frequency to produce a frequency drift signal representing undesired frequency drift in said excitation frequency, electrical circuits to compare the signal from said balance discriminator with the frequency drift signal to produce a motor-control 10 signal, and follow-up motor means for adjusting said capacitors and to which said motor control signal is applied.

15. A system according to claim 14 in which each or said adjustable capacitors is provided with a respective follow-up operating motor, circuit connections for applying the balance signal from said balance discriminator to operate one motor without applying said balance signal to electrical circuits, and circuit connections for applying said motor control signal to operate the other follow-up motor.

16. A system according to claim 14 in which said motors adjust said capacitors simultaneously in opposite senses.

ROBERT RAY WEEKS.

No references cited. 

